So the British royal family will need to stop doing this now.
Practically every royal ruler during WWI was a descendant of Queen Victoria, and she died in 1901.
Also, in Jane Austen’s last novel, “Persuasion,” Anne Elliott, the protagonist, almost marries her first cousin, but decides against it, not because he was her first cousin, but because he was a con man, and she loved Captain Frederick Wentworth more, anyway.